The invention relates more particularly to mixing grids of the type comprising at least two sets of crossed plates fixed together at their crossover points, delimiting cells some of which are for receiving fuel rods and the other are for receiving guide tubes, the grids being provided with coolant-stirring fins extending the plates downstream and disposed to deflect the coolant transversely to its general flow direction, each plate being provided with abutment means projecting inwards from each of the faces of the cells for receiving fuel rods.
The invention is particularly applicable to fuel assemblies in which the support structure includes guide tubes interconnected two end fittings and, in addition to mixing grids of the above kind that do not participate in supporting the rods (i.e. to supporting them vertically), at least one additional grid for carrying the rods. For this purpose, the grid is provided with springs cut out from the plates or fixed to the plates and designed to urge the rods against abutment means constituted by bosses situated opposite.
Development in reactor characteristics, in particular towards higher thermohydraulic performance and combustion rates gives rise to the use of grids other than supporting grids having abutment means delimiting passages that are larger in size than the rods, thereby facilitating coolant flow and avoiding damage to the rod sheaths during insertion. This clearance is small enough to limit rod vibration to an amplitude smaller than that which could give rise to the sheath being damaged and to the stirring fins being hammered by the rods.
The present invention seeks in particular to provide a grid of the above type in which the abutment means participate in making the temperature within an assembly more uniform, without giving rise to excessive headloss.